Internal combustion engines generate exhaust as a by-product of fuel combustion within the engines. Engine exhaust contains, among other things, unburnt fuel, particulate matter such as soot, and harmful gases such as carbon monoxide or nitrous oxide. To comply with regulatory emissions control requirements, engine exhaust must be cleaned before discharge into the atmosphere.
Engines typically include after-treatment devices that remove or reduce harmful gases and particulate matter in the exhaust. For example, a diesel engine can be equipped with a diesel oxidation catalyst (DOC) that promotes oxidation of unburnt fuel, carbon monoxide and/or nitrous oxide, and a diesel particulate filter (DPF) that traps particulate matter. Over time, the increasing volume of trapped soot impedes the flow of exhaust through the DPF and degrades engine performance. One commonly used technique for in-situ cleaning or regeneration of a DPF involves raising the temperature of the DPF above a combustion or oxidation threshold of the soot particles accumulated on the DPF. In most cases, this is achieved by heating the exhaust before it enters the DPF. When the hot exhaust interacts with the soot particles, they oxidize.
The temperature of exhaust flowing through a DPF can be raised in many ways. For example, engine operating parameters such as the fuel-air mixture composition or engine load can be varied to produce exhaust having a higher temperature. Alternatively, fuel can be injected directly into the exhaust and oxidized in the presence of the DOC at a location upstream of the DPF to raise the temperature of the exhaust. In this arrangement, the DOC, together with the fuel injectors or dosers, acts as an exhaust heater.
A DOC typically becomes active, however, only above a threshold temperature, known as the DOC light-off temperature. When a temperature of the exhaust exceeds the DOC light-off temperature, the DOC promotes oxidation of fuel injected in the exhaust via an exothermic reaction. At low engine loads, however, the temperature of the exhaust may remain below the DOC light-off temperature. In such cases, to activate the DOC, it may be necessary to pre-heat the exhaust before it interacts with the DOC.
One attempt to address the problems described above is disclosed in U.S. Patent Application Publication No. 2011/0047973 of Wilhelm et al. published on Mar. 3, 2011 (“the '973 publication”). In particular, the '973 publication discloses a particulate trap regeneration system, which includes multiple after-treatment branches. Each after-treatment branch of the system of the '973 publication has a dedicated hydrocarbon doser and one or more particulate traps. In addition, the system of the '973 publication has a controller to control the amount and duration of fuel injection in each after-treatment branch. The system of the '973 publication also includes a regeneration event synchronization module to synchronize the regeneration events in the multiple after-treatment branches.
Although the system of the '973 publication discloses more than one doser, each after-treatment branch includes only one doser, which injects fuel at one location in the after-treatment branch. Because the system disclosed in the '973 publication utilizes a single doser that injects fuel in one location, the injected fuel may not mix well with the exhaust flowing in the associated after-treatment branch. Oxidation of the fuel in such a non-homogeneous mixture may cause non-uniform heating of the exhaust. The resulting temperature gradients may induce thermal stresses in the particulate filter and/or an associated oxidation catalyst, causing them to break or be damaged.
The exhaust system of the present disclosure solves one or more of the problems set forth above and/or other problems in the art.